In recent years, electronics devices, particularly such portable information devices as cellular phones, laptop personal computers and camcorders, have improved in performance and become popular, creating greater demand for small and lightweight secondary batteries whose energy density is high. A study of more advanced batteries is under way. In particular, a lithium ion secondary battery, one of such secondary batteries, is gaining attention.
The lithium ion secondary battery is made in the following manner: Positive electrodes, where positive electrode active material layers are formed on positive electrode current collector surfaces, and negative electrodes, where negative electrode active material layers are formed on negative electrode current collector surfaces, are stacked via separators made of a porous synthetic resin film and impregnated with an electrolytic solution before being turned into battery elements. The positive electrode active material layers are made of positive electrode active material powder, such as lithium cobalt composite oxide, conductive powder, and binder; the positive electrode current collector surfaces are made of aluminum foil. The negative electrode active material layers are made of carbonaceous negative electrode active material powder and binder; the negative electrode current collector surfaces are made of copper foil.
In order for batteries to serve as a power source for electric vehicles or the like, it is necessary for many unit cells to be connected in series and parallel depending on the required electric capacity.
FIG. 6 is a diagram illustrating an example of a conventional battery pack.
The battery pack shown in FIG. 6 is a large-capacity lithium ion battery pack consisting of seven unit cells. In a first casing portion 3a and second lid portion 3b of the battery pack 1, a protection circuit board 7 is mounted on an assembled battery 4 in which unit cells with a capacity of about 4.0 Ah are arranged in a 7-series, 1-parallel type; the battery pack 1 is equipped with a connection connector 8 for external connection.
FIG. 7 is a diagram illustrating another example of a conventional battery pack. The battery pack shown in FIG. 7 is a large-capacity lithium ion battery pack consisting of 14 conventional unit cells.
In a first casing portion 3a and second casing portion 3b of the battery pack, unit cells with a capacity of about 1.0 Ah are arranged in a 7-series, 1-parallel type to produce assembled batteries 4a and 4b, which are then connected in parallel so as to form the arrangement of a 7-series, 2-parallel type. In addition, a protection circuit board 7 is mounted; a connection connector 8 is provided for external connection.
For such battery packs, a protection circuit board is mounted in each battery pack to protect the battery against abnormalities during a charge and discharge process of the battery. Depending on the volume of the space occupied by the protection circuit board, the volume energy density decreases. For example, see Patent Document 1.
The production and maintenance of a battery pack in which unit cells are connected in series or parallel or in both series and parallel become more complex as the number of the unit cells increases.
What is known as a substitute for a battery pack in which unit cells are directly disposed is a battery pack that has, in a case, a plurality of battery modules in which a plurality of unit cells are stored.
When battery modules are used, depending on how various battery modules are combined, it is possible to make more efficient a process of assembling battery packs with various levels of electric power output.
Even in the battery modules, as in the case of the battery pack, a protection circuit board is provided for an assembled battery in which unit cells are connected in series or parallel or in both series and parallel. For example, see Patent Document 2.
For the protection circuit board, in general, various semiconductor elements are used. The semiconductor elements having a voltage and current resistance are used that are appropriate for the battery modules.
For example, suppose that, in order for a battery module with an output of 24 V to withstand the voltage, a protection circuit is made up of elements that withstand double the output voltage, i.e. 48 V. If the two modules are connected in series, there is no room left for voltage resistance. If the three modules are connected in series, the output voltage then exceeds the voltage that the elements can withstand; when abnormalities occur, the protection function may not work as the semiconductor elements break down. Accordingly, there is a limit to connecting battery modules together electrically.
There is also a method by which a protection circuit board is made up of components that withstand high voltages. However, the problem, among other things, is that the external dimensions of the components are large and that the components are expensive per unit. The components are unfavorable in terms of both volume and prices.